Of unknown origin.
It is a created name.
Ygritte is a character name created by George R.R. Martin, for the “Song of Ice and Fire” novels and “Game of Thrones” TV series.
In the novels, Ygritte is described as a red-haired spearwife, with great skills as an archer.
In the following video you may watch a short tribute to the character (Game of Thrones spoilers included):